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Critical Currents in the Superconducting Surface Sheath
Phys. Rev. 149, 186 – Published 9 September, 1966
DOI: https://doi.org/10.1103/PhysRev.149.186
Abstract
Results are presented of magnetization measurements on Pb-Tl cylinders in axial magnetic fields of such magnitude that superconductivity is confined to the superconducting surface sheath. Specimens whose Ginzburg-Landau values range from 0.6 to 1.4 have been investigated. For these cylindrical specimens a nonzero magnetization exists beyond the upper critical field up to a field (1.7 to 1.75 ) identifiable with the surface nucleation field (1.695 ). Evidence is presented that indicates the observed magnetization results from partial shielding of the interior of the cylinders by critical currents induced in the multiply-connected surface sheath. Our experimental results are in quantitative accord with recent theoretical calculations of Fink and Barnes concerning the influence of critical sheath currents on the effective bulk-magnetization characteristics of a cylinder. It is observed that diamagnetic () and paramagnetic () shielding occur in uniformly increasing and decreasing fields, respectively, which is consistent with the existence of a multiconnected superconducting region which can carry a total current. The surface localization of the superconductivity involved is demonstrated by the proximity effects of electroplated normal metals on the specimens. Cu plating reduces both the magnetization and (∼1.4 ). Owing, perhaps, to their magnetic properties, electroplated Ni and Cr have a much more drastic effect than electroplated Cu. To the limit of experimental sensitivity, all traces of superconductivity above are removed by Ni and Cr platings.
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